The atmospheric surface layer extends from the ground to a few tens of meters in height. The surface layer is unstable if heat is rising from the ground, as is common during sunshine. The rising heat adds to the turbulence. The surface layer is stable if the ground is cooler than the air so that heat is transferred downward, as is common at night. Such stability tends to damp turbulence. The surface layer is near neutral if wind shear produces almost all of the turbulence, whereas the heat transfer between ground and air is not effective in producing or damping turbulence. This condition typically occurs when the wind is strong and/or solar radiation is weak. Heat flux is the vertical transport (rising or descending) of heat from or to the ground. Momentum flux (always downward) is the effect of the frictional drag of the ground on the wind blowing over it.
Temperature fluctuations in the air cause the scintillation (twinkling) of distant light sources. The strength of these temperature fluctuations at the spatial scales (10 cm to 1 mm) that produce scintillation is represented by the refractive-index structure parameter, which is denoted by C.sub.n.sup.2. The smallest spatial size of temperature fluctuations is represented by the inner scale of turbulence, denoted by l.degree.. Horizontal homogeneity means that the average quantities (as opposed to instantaneous turbulence fluctuations) are the same everywhere in a horizontal plane to within distances of several hundred times the height above ground; examples of such average quantities are heat flux, momentum flux, average wind speed, C.sub.n.sup.2, and l.degree.. For the horizontally homogeneous atmospheric surface layer there are empirical relationships between the fluxes of heat and momentum and the parameters C.sub.n.sup.2 and l.degree.. The discussion below refers to this horizontally homogeneous case.
A variety of in situ techniques are known for measuring atmospheric fluxes. The prior art methods of measuring fluxes include using a sonic anemometer or performing indirect-dissipation measurements using a fine wire thermometer and hot-film anemometers. These prior art methods, however, require excessive processing and high data rates. Also, the instruments are not particularly rugged. For example, fine-wire thermometers are subject to hygroscopic-particle contamination.
A problem with the prior art relates in situ measurement of fluxes at a single point. Point measurements of fluxes are extremely location dependent. Movement from one location to another produces different readings. To obtain more general readings, for example as part of an ecological or air quality study, it is required to take measurements at a large number of points and to average the resultant data, making the overall collection of data rather cumbersome.
A measurement of scintillation can give C.sub.n.sup.2, which is, in turn, sufficient information to determine heat flux for temperature fluctuations is represented by the inner scale of turbulence, denoted by l.sub.0. Horizontal homogeneity means that the average quantities (as opposed to instantaneous turbulence fluctuations) are the same everywhere in a horizontal plane to within distances of several hundred times the height above ground; examples of such average quantities are heat flux, momentum flux, average wind speed, C.sub.n.sup.2, and l.sub.0. For the horizontally homogeneous atmospheric surface layer there are empirical relationships between the fluxes of heat and momentum and the parameters C.sub.n.sup.2 and l.sub.0. The discussion below refers to this horizontally homogeneous case.
A variety of in situ techniques are known for measuring atmospheric fluxes. The prior art methods of measuring fluxes include using a sonic anemometer or performing indirect-dissipation measurements using a fine wire thermometer and hot-film anemometers. These prior art methods, however, require excessive processing and high data rates. Also, the instruments are not particularly rugged. For example, fine-wire thermometers are subject to hygroscopic-particle contamination.
A problem with the prior art relates to in situ measurement of fluxes at a single point. Point measurements of fluxes are extremely location dependent. Movement from one location to another produces different readings. To obtain more general readings, for example as part of an ecological or air quality study, it is required to take measurements at a large number of points and to average the resultant data, making the overall collection of data rather cumbersome.
A measurement of scintillation can give C.sub.n.sup.2, which is, in turn, sufficient information to determine heat flux for the very unstable case. One of the early demonstrations of this by Wyngaard, J. C., J. C. Kaimal, G. R. Ochs, R. J. Hill and D. C. Sorensen, "An optical heat flux experiment," Fourth Symposium on Meteorological Observations and Instrumentation, 10-14 April, 1978, Denver, Colo., published by AMS, Boston, Mass., used a laser source. Since then, the large-aperture C.sub.n.sup.2 -scintillometer has become the standard optical instrument for measuring C.sub.n.sup.2 ; it is far superior to using a laser source. This instrument has not been applied to measuring heat flux under very unstable conditions. This instrument is described by Wang, Ting-i, G. R. Ochs, and S. F. Clifford (1978), "A saturation-resistant optical scintillometer to measure C.sub.n.sup.2," Journal of Optical Society of America, Vol, 68, pp. 334-338; and Ochs, G. R. and W. D. Cartwright (1981), "An optical device for path-averaged measurements of C.sub.n.sup.2," Proc. SPIE, Vol. 277, Atmospheric Transmission, 21-22 April, 1981, Bellingham, Wash. 98227. The general possibility of measuring heat and momentum fluxes (and even humidity flux) by means of scintillation was described by Hill, R. J. and G. R. Ochs (1983), "Surface-layer micrometeorology by optical scintillation techniques," Technical Digest, Optical Techniques for Remote Probing of the Atmosphere, 10-12 January, 1983, Incline Village, Nev. They noted that a scintillation measurement C.sub.n.sup.2 and .sup.l.sub.0 would give heat and momentum fluxes for any stability condition (i.e., unstable, near neutral, stable). The only exception is over surfaces having such great humidity flux that water vapor provides the instability rather than heat. Hill and Ochs did not foresee present methods of actually measuring l.sub.0 and therefore obtaining the fluxes. Perhaps the first method for measuring l.sub.0 by means of scintillation was given by Livingston, P. M. (1972), "Proposed method of inner scale measurement in a turbulent atmosphere," Applied Optics, Vol. 11, pp. 684-687. This method involves using laser sources and differing propagation path lengths; it has never been attempted and is probably impractical. The first attempt to use scintillation for measuring l.sub.0 and C.sub.n.sup.2 used three large-aperture C.sub.n.sup.2 -scintillometers having different aperture sizes; this was performed by Hill, R. J. and G. R. Ochs (1978), "Fine calibration of large-aperture optical scintillometers and an optical estimate of inner scale turbulence," Applied Optics, Vol. 17, pp. 3608-3612, who concluded that the accuracy was too poor to provide the fluxes of heat and momentum. Ochs, Gerard R. and Reginald J. Hill (1985), "Optical-scintillation method of measuring turbulence inner scale," Applied Optics, Vol. 24, pp. 2430-2432 (hereinafter referred to as "Ochs et al.") succeeded in measuring l.sub.0 by using one large-aperture C.sub.n.sup.2 -scintillometer in combination with a laser source and small-aperture receiver. Because of the system parameters, the C.sub.n.sup.2 -scintillometer, by itself, could not provide a high-precision value of C.sub.n.sup.2, so Ochs et al. did not foresee that this basic design might be adapted to determining heat and momentum fluxes. Hill, Reginald J. (1988), "Comparison of scintillation methods for measuring the inner scale of turbulence," Applied Optics, Vol. 27, pp. 2187-2193, gave a theoretical comparison of the method of Ochs et al., with yet two more possible methods of measuring l.sub.0. One of these two new methods, the bichromatic correlation of irradiance method, was tested experimentally by Thiermann, Volker and Ehud Azoulay (1989), "Modeling of structure constant and inner scale of refractive-index structure parameter fluctuations--an experimental investigation," Proc. SPIE Technical Symposium on Aerospace Sensing, Conf. 1115, Propagation Engineering, 27-31 March, 1989. This test gave good values of l.sub.0 and C.sub.n.sup.2 ; they did not attempt to deduce the heat and momentum fluxes.
From the above discussion it is clear that, although there has been some theoretical discussion, a need exists in the art for a practical system for scintillation measurement of atmospheric fluxes, such as heat and momentum fluxes.